1. Field of the Invention:
This invention relates generally to full motion video transmission over communication channels, and more particularly to such a system which digitizes the analog video and compresses the data for transmission.
2. Description of the Prior Art:
Color television video transmission uses a complex analog signal which is broadcast over the air. The video signal has components in it that control red, green and blue "guns" located in the television receiver. The receiver screen is divided into a large number of points, called pixels, which the red, green and blue guns fire against. The intensity of the color from each gun depends upon the video signal, and when mixed at each pixel, defines the desired color for the screen at that particular point. The guns sweep horizontally across the screen line by line until an entire frame is completed. Normally, there are about thirty frames per second.
Efforts are being made to send video signals in digital form over communication channels for telephone conferences and the like. The communication channels may be telephone lines or local area networks. The analog video signal can be digitized into a digital word for each pixel. The digital word or number will have components therein to control the relative intensity of the red, green and blue guns. These digital numbers can be transmitted through a modem of a control computer to a receiver for display. These digital signals could also be stored on a disk for playback. However, there will be an extremely large number of bytes to transmit in a very short space of time. There can be from 50,000 to 200,000 bytes per frame, and normally thirty frames per second are transmitted in conventional television broadcasting. There are two fields that make up a standard video frame, commonly called an odd field and an even field. It sends the information in one field first, and next the other field, which comprises in between lines. Existing communication channels, which may handle between 56K baud (56,000 bits per second) and 1000K baud, cannot handle that rate of transmission. The amount of bits would also require an excessive amount of storage space if stored on disks.
Often, much of the television frame changes little from frame to frame. Particularly in telephone conferencing, there would be normally a constant background. Efforts are now being made to transmit full motion video, but introducing only a portion of the signal to lower the number of bytes that must be transmitted for each frame. There are several methods. One method divides the screen into many small sections, and through extensive processing, gives priority to the sections with the most severe movement. Other methods merely slow the frame rate, resulting in a jerky picture. The equipment is expensive, or the picture quality is poor. The prior art systems are inflexible and they cannot adapt to various transmission rates to take advantage of higher data rates allowed on some systems than on others.